How We Designed Our Solar Greenhouse

We live and garden on an urban lot in Calgary, Canada, located on the 51st parallel north and approximately 80 km east of the front ranges of the Canadian Rockies. This northern climate presents many design challenges, including less than one hundred frost-free days, an annual mean temperature of 4.1 degrees Celsius and summer cyclonic weather patterns (i.e. high risk of hail). We are also considered to be a moderate temperate desert as our precipitation is around 500mm including snow. However, one of the advantages of growing food up north is the long summer days. There is no better place to observe this than in Alaska which also has an average of 100 frost free days but is renown for growing the largest vegetables in the world. Also, despite being cold in the winter, it is rarely overcast and we enjoy mostly sunny days. These two factors combined result in Calgary having nearly the same solar potential as Florida.

Traditional European Greenhouse

Passive Solar Greenhouse

In good ol’ permaculture fashion, we set out to enhance sectors and conditions that would improve our growing season (sunlight, heat) while minimizing those that we considered detrimental (cold, hail, frost). We quickly determined that a passive solar greenhouse was just what we needed and we set out to design one for our backyard.

There are two major considerations when designing a greenhouse: heat and light. Interestingly, the traditional European-style greenhouses were developed in the low countries of northern Europe as a response to low level of predominantly diffuse light prevalent in the winter time (think cloudy, overcast winters). This design was brought to this continent with little consideration for the differences in climate and latitude encountered here.

A greenhouse that is better suited to our winter conditions is a passive solar greenhouse. These greenhouses are designed to accept and enhance the direct sunlight and heat from the south while preventing heat loss by insulating the north, east and west sides.

Just this past December our greenhouse went up. Although currently empty (we still have to design out and build the interior) we’ve been receiving a ton of interest and questions about how we designed the structure itself. So, here are some photos (including a nifty tim elapse slide show of the construction) and a brief description of the structure.

The General Design

We chose to site the greenhouse on the concrete pad behind the house (originally designed as a parking pad — who needs all that parking space anyways?). The size of the greenhouse is 10’ x 20’ with a 10’ ceiling (3m x 6m x 3m) which covers nearly half of the cement pad. Generally, this style of greenhouse works best if it is twice as long as wide. We also designed a shed style roof with an overhang to capture rain and reject some of the overhead summer sun.

The Structure

The entire building (with the exception of the glazing) is
made from structurally
insulated panels.

The building is made from structurally insulated panels (SIPs). These pre-fabricated panels consist of an insulating layer (styrofoam) sandwiched between two layers of structural board. Although the idea of building the greenhouse from natural materials (i.e. straw & cob) was very appealing to us — the truth is that I did some engineering consulting for the SIP panel manufacturer which resulted in getting a sweet deal on the building materials. There are also some great advantages to using SIPs — they are mold and rot proof, which is very important in humid environments. These panels are fire proof and do not off-gas. They are highly insulative with an average R-value of 25 (better than most homes) and because the panels are pre-fabricated, the main structure itself went up in less than one day.

In the end this greenhouse is going to produce far more energy in its life than it consumed in its manufacture. Every calorie of food that is supplies to my family is 10 – 25 that do not have to be expended in the industrial system when you consider tractors, fertilization, pesticides, shipping, refrigeration and transport. With a life expectancy of at least several decades we are quite satisfied with the energy payback.

Glazing

The angle of the glazing from
horizontal is an important
design consideration

Glazing on a greenhouse is the surface that lets the light in — usually glass or plastic sheets. Having the glazing at an angle allows us to maximize winter sun (increasing heat in the winter) and minimize the summer sun (reducing overheating in the summer). The angle of the glazing from horizontal is an important design consideration and the optimal angle depends on which part of the season you want to do most of your growing.

As a rule of thumb, to optimize the glazing angle for winter growing, take your latitude and add 15 degrees. In our case the optimal angle would have been 51 + 15, or 66 degrees. However, as long as the glazing angle is within 45 and 75 degrees you will be within 5% of optimum — therefore it often makes more sense to design the building to height restriction and material constraints vs optimal glazing angle. In our case, the actual glazing angle is 55 degrees.

For glazing we chose to use triple glazed polycarbonate with an R-value of 2. This is dramatically less insulative than the walls (R-25) and so to keep the heat in we are going to use an insulated draw-down curtain which will be drawn at night and raised in the morning.

What’s Left To Do (there’s lots!)

With the structure and glazing up, our big project over the next few months is to complete the interior features. There are quite a few other considerations, here’s a brief description of our plans at this time:

Heat Retention & Rejection

Low and high vent holes have been cut out to
provide passive ventilation

Typically, in passive solar building design, the recommended percentage of glazing to prevent overheating is 7% – 12% of the total southern wall surface. For instance, if your southern wall was 100 m2, you should have only 7 to 12 m2 of windows. If you go above 12% you have to add additional mass in the building to absorb the incoming solar energy. If you go above 20% you are going to overheat your building.

Well, our greenhouse has 90% glazing coverage on the south surface. This amount of glazing is required to capture sufficient energy in the cold months to keep the space warm but is setting us up for potentially major overheating issues in the summer. There are a couple of strategies to deal with overheating which we intend on employing: (i) heat retention and (ii) heat rejection.

For heat retention, we plan to install six inch non-perforated weeping tiles below the raised garden beds. These “earth tubes” will receive hot air directed from the ceiling of the greenhouse using a small solar-powered fan. Effectively, we will be storing surplus heat in the soil of the garden beds.

For thermal mass, we will use black containers of water along the back wall. We are also considering installing some cob features to soak up additional heat, however we are a little concerned about the cob being exposed to high humidity — I’d be interested to know if anyone has experience with this.

For heat rejection we have cut out multiple air vents, both high and low, which will be operated with powerless wax-driven arms (here’s a link to a similar product). A rule of thumb for sizing ventilation is to have the total venting area equal to 25 to 30 percent of the total area of glazing. We may also need to install a shade cloth under the front eave in the summer to further reduce the heating load. Time and experimentation will tell.

Auxiliary Heating System

When we get to the inevitable -30°C day with cloud cover the greenhouse is going to need some extra heat. Our plan is to build a rocket stove back-up heating system. While driving through the Calgary industrial park several weeks back I was amazed at the amount of good wooden pallets that were being disposed of. When I stopped to talk to one small business, the owner pleaded for me to take them away. And so, these pallets will serves as fuel for the rocket heating system and will also make great building material. Sawdust left from the processing will be used for our composting toilets and mulch.

Aquaponics System

I am very interested in experimenting with combined fish and hydroponics systems — and the greenhouse will provide just the space I need. The added beauty of combining an aquaponics with the solar greenhouse is that the aquaponics system will increase the thermal mass while providing a bounty of fish and veggies. However an important consideration will be the increased humidity. While condensation & rot is not a concern with our mold-proof structure, most plants do grow best at a relative humidity between 45 and 60 percent. Leaf rot and flower, fruit and stem diseases increase in very high humidity environments.

In Summary

The solar greenhouse is certainly going to make a big re-appearance here in our climate (and similar climates) as people start to re-connect with their food and desire more local and sustainable ways of nourishing themselves.

We are very excited about working on the interior design and construction over the remainder of the winter and have already been perusing specialty seed catalogues looking for appropriate banana and fig tree varieties. There is a steep learning curve ahead as we expand our gardening knowledge with new plant varieties, techniques, time and space stacking and generally about greenhouse operation. But we are keen and eager and look forward to sharing our successes… and failures too!

Stay tuned for updates….

Rob and Michelle Avis are Mechanical Engineers and Permaculture Designers and run their business, Verge Permaculture in Calgary, Alberta, Canada.

22 Responses to “How We Designed Our Solar Greenhouse”

Peter Willis

Hi Rob
Great to see you doing this in Calgary! I built a passive greenhouse for my parents here in New Zealand last Autumn. Nothing quite like yours as our climate is far more mild. I need to alter it a little as it worked very well in the Winter but did not let in enough light in Summer to use the whole grow-beds. We had good success with stacking, having potted veges growing at chest height above soil beds. Moisture was a concern and I need to try a couple of things this year to lessen it, we had no rot or problems just more than I’d like. That said we do have high humidity in Auckland year round so it may be a hard fix.

I live near a rammed earth house and have visited the owner a few times, they say that despite having only single glazing they never have condensation on the windows. The theory is that the walls regulate the condensation, absorbing it when high and releasing it when it drops holding the house in the 40-60% band year round. This has also been achieved by an earth builder further north but he did it on an existing structure by plastering a cob mix over chicken wire onto existing drywall inside the house.

Another idea I have seen is an extension of the weeping tile idea you mentioned where perforated 6 inch flexible plastic drainage pipe is installed in the soil beds and piped back to a central plenum (or several) then a fan is used to exchange the greenhouse and soil air every 15-20min. This causes the air moisture to be condensed onto the surface of the pipework and the latent heat in the air moisture is given up the soil beds. Much more detail can be found here, a bit of work at the start but big benefits in a cold climate.
Have a look at http://www.sunnyjohn.com/indexpages/shcs.htm

I also wondered in your climate if you fitted a hollow pipe vertically on the back wall that vented at each end to the outside air if this would act as a condensation point for moisture in the glasshouse (since the interior of the pipe would be cold air from outside). Then the water condensing on the pipe would run (hopefully) down and into a drain or bucket for use in the beds. Possibly it would get too cold and just ice up. Maybe the pipe should be plastic instead (more insulating), or insulated with pipe wrap in the coldest months, whacked with a hammer every day to dislodge the ice??? It barely even frosts in my climate so I have no experience of how this might behave :(. Trouble with pioneering is no one can tell you for sure what will work!

Thank you for much useful information, I’m sure I’ll have to go back to this article one day! I had to check and found I live on the 67st parallel north, and the annual mean temperature here is 3.9 degrees Celsius down by Lake Mjøsa. This is quite similar to your place.

I add some pictures in the URL from my trip to Northern Norway past summer, from where there is sun 24 hours a day. Surely the vegetables become extremely tasteful with all that light!

Cam Wilson

Hi Rob

Great work and glad to see your mother in law is already letting you renovate the house!

Rob touched on an area where he is very knowledgeable, that of energy accounting. Definetely something worth getting your head around, and can lead to some interesting choices such as the pragmatic one Rob made regarding materials. Maybe you could write a little emergy article some time down the track Rob (when you get a spare moment).

A very nice job on your passive solar greenhouse. As a person who has desinged, built and taught passive solar greenhouses since the early 1980’s, I would add to your two major considerations a third consideration when designing a greenhouse: coolth. Your coolth is provided by the south face overhang.

I do greenhouse consultations here in Northern California, US. You would not believe how many greenhouses I am called in to look at that were designed for heat and light in the winter, and totally overheat even in the springtime, killing young seedlings.

Many people who have built greenhouses, especially from kits, suffer badly from the lack of summer shade and from enough summer venting. Many of these also gobble lots of fossil fuel to heat in the winter. Having a greenhouse with total glazing on all sides and on all parts of the roof indicates a lack of understanding of the movements of the sun.

Also, many greenhouses I have seen have condensation problems, even ones attached to rammed earth houses, which were originally developed in South Eastern Arizona, where I was originally trained in Passive Solar Greenhouses.

Eric

Hi Rob,

Great article and very nice design on the greenhouse. I like the sketchup rendering.

I have heard that pallet wood is typically treated with formaldehyde among other nasty chemicals. I don’t suppose this would be so much of an issue if used as fuel, but be cautious about using its sawdust in mulch as you surely wouldn’t want chemicals contaminating your crops. I’m not certain of the likelihood of leaching, but I just wanted to throw that info out there.

Bonnie Lester

Wow, Rob you have done more for Permaculture in Canada in the short time you have been working.
You are a real inspiration,I was getting scared that Canada would be left in the permaculture dark ages because of all the do nothing posers we have over here,charging high prices and inflating their experience and resume.
You are unique and we are sure lucky to have you,congratulations and thanks for all the inspiration.
Bonnie Lester

Thanks for this, and others for posting the other great links. We are in the process of designing ours here in Detroit, MI USA where its cold, but not as cold as Calgary. We’re using insulated tempered glass for our glazing that we got very inexpensively from the back room of a local window shop. We were thinking using natural plaster (lime?) covered earth bags and wood for the framing. Anyone out there have experience with this?

Thanks for the comment. I think we need to take a pragmatic approach to the materials that we use. We are in a transitory time and things are moving very quickly, I prefer to look materials invested through the lens of how much energy can the design saves in calories over the industrial food system. With a food system that consumes anywhere from 10 – 86 calories of energy for every calorie of food consumed this greenhouse can pay for itself very quickly. Furthermore in my climate we have just over 100 frost free days, so eating in Calgary is an energy intensive habit. With 1 million people in the city all eating around 2000 calories/day that represents close to 13,000 bbls of oil per day.

Ultimately I think you have raised an important point, should we be living on a typical modern diet in a climate as harsh as the one I live in? Probably not. This climate supports a diet largely based in berries and meat with a limited amount of greens and veggies.

Thanks for the comment. The problem with greenhouses is that the plant transpire a lot more than a few humans so condensation is much more of a problem. I would like to try this same design in an earth sheltered version and use the soil a humitidy regulator. My friend has some large 1000 l bags used for soil that would make a quick wall and allow breathing. This system is going to be a lot quicker than using tires.

Harold Brown

Are fig trees a good return on greenhouse space? Food production is an exercise in self suffiency or is your solar greenhouse just an expensive toy to make you greener than your peers? Real informarion and practise for production and sustainability of a family of six would prove to be usable information. As all of North America falls into a third world socialist economy, your information on greenhouse agriculture could prove an incredably source of valuable survival planning. This is especially true for you as you have built an intelligently designed high quality solar greenhouse. Would you recommend a growing systen like square foot gardening or a different growing systen. Your output of infoemation would turn you into a solar greenhouse Gruru and a hero of info to many people.
I did enjoy your site and thank you!

Glad to see you project and hope that it will prove successful in your climate. I was looking at your greenhouse when trying to give someone an idea of a passive solar greenhouse and had two questions I have to ask. Why did you not back/connect the greenhouse to the house, and with the placement attached or not why did you slope the roof to drain water toward the house? (are you going to catch any snow/rain run off into a containment system? The design is a stand alone design which gives you that pitch. I have also taken Elliott Coleman’s workshop on four season gardening and he found that in his colder climate the condensation on the plastic would freeze and help with the temperatures in the greenhouse, he also found that an extra row cover or plastic on the plants themselves gave another 1 1/2 zones to the south. He also found plants that did well in the uncooled greenhouses, and small hoop houses with straw insulation for plants rootcellaring over winter in the ground. And Will Allen uses big compost piles in the corners of his hoophouses to give winter heat, and the aquaponics to help stabilize the temperatures. It will be nice to see how your added heat traps/creation help on those colder days.

Mark Carlson

Thank you so much for the information. I am currently in the process of building my own thermal banking greenhouse. However, I am also going to use geothermal to heat the greenhouse in the winter and to control the temperature of the water. I live in cold Wisconsin, and I think the geothermal will be efficient. S&B Geothermal is doing the geothermal install, and have been very helpful in both keeping the install cost low, and in expertise on the geothermal. Any advice you could give me on the design of the greenhouse itself would be helpful. I am basing the design on the http://cookingupastory.com/sustainable-energy-thermal-banking-greenhouse-design.